1. Field of the Invention
The present invention concerns, in general, a measurement device and method for performing a voltage differential measurement. Particularly, it concerns a device and a method for measuring the effects of contact resistance between a test circuit board and a populated printed circuit board or electronic module to determine if the contact resistance is sufficiently low for testing purposes.
2. Description of Related Art
Contamination on conductive interconnecting contacts can cause significant signal degradation when measured through the contacts. This degradation is a result of contact contamination build-up, commonly referred to as contamination resistance. Contamination is usually found in the formation of a film or layer of material on the contact or plated surface of the contact. Contamination resistance is the electrical resistance caused by various types of contamination films and materials which may become disposed on the electrical contacts and ultimately decrease the conductivity between the contacts.
Importantly, when performing sensitive voltage and current test measurements on Integrated Circuits (ICs), contamination resistance on the interconnecting contacts can lead to incorrect measurements and false failure designations due to a drop in voltage across the contamination resistance or current limited by the same.
High speed microprocessors typically draw large currents. Thus, when these modules are under test, contamination on the interface board I/O pins can provide enough series resistance to reduce the Vdd voltage at the device and increase the ground voltage reducing the device speed. This results in microprocessor devices being sorted into a lower speed category than they are otherwise entitled to be in.
In order to address this problem, it is common to periodically interrupt testing to measure the contact resistance during testing. If such tests reveal that the contact resistance is increasing, contacts are cleaned and testing is then resumed.
Generally, two measurements are made for measuring contact resistance: a 2-terminal measurement and a 4-terminal measurement. In the 2-terminal measurement, a small known current is forced through the circuit and the associated voltage drop is measured. The total resistance, including resistance of both contacts and circuitry therebetween, is then calculated from the voltage to current ratio. In the 4-terminal method, a known current is forced through two outside terminals of a device and a voltage is measured across two inside terminals on the device. The ratio of measured voltage to forced current gives the resistance of the circuitry alone. The difference between these two measurements is the contact resistance. Variations on these measurement techniques have been used to measure the contact resistance value where a known current or voltage can be applied. However, there has been no method of measuring contamination build-up without interrupting the normal operational testing of integrated circuits to provide the known current or voltage. Nor has there been a method of detecting contamination build-up just as it occurs.